(1) Field of the Invention
The present invention relates to a cryogenic technique and, more particularly, to a method for sealing joints of pipeline and containers operating with cryogenic fluids at extremely low temperatures. The invention may find application in aerospace, aircraft, nuclear equipment, ship building, and other industries.
(2) Description of Related Art
Basic connections used in containers for storing or in pipelines for transporting cryogenic fluids are flange joints. Improvements of reliability in flange joints of aforementioned containers and pipelines is a matter of great importance that nowadays attracts attention of scientists and designers.
Two lines of work are in progress: improvement of sealing units is attained either due to special features of the flange joint designs (RU 2145687, F16L 23/00, F16L 19/00, F16L 21/00, published Feb. 20, 2000; RU 2137023, F17C 13/00, published Sep. 10, 1999; Application RU 92011930, F16L 19/02, published Aug. 27, 1995) or due to the use of new sealants put on the internal surface of flanges (RU 2227240, F16L 15/04, published Apr. 20, 2004; RU 2170751, C09K 3/10, C09K 21/02, published Jul. 20, 2001; RU 2163917, C09K 3/10, F16L 15/04, published Mar. 10, 2001; RU 2194734, C09K 3/10, published Dec. 20, 2002).
Common disadvantages of the above methods of sealing are their complexity, high cost, and inefficiency of application for pipelines operating with cryogenic products.
In accordance with a conventional practice, all aforementioned flange joints are normally assembled and tightened, including tightening with the use of threaded fasteners, prior to loading of the container or pipeline with a cryogenic fluid, i.e., at ambient temperature. However, even at the very initial stage of use, i.e., when the container or pipeline is filled with a cryogenic fluid, seals may be subject to a failure due to mechanical stress resulting from sharp cooling of connection flanges. Such failure may be accompanied by a strong leakage and evaporation of cryogenic liquids (it is visually observed in air: the joints begin to “smoke”). Taking into account the fact that at cryogenic temperatures metals loose their plasticity, it is impossible to eliminate the leakage by additionally tightening the flange bolts, since overtightening may cause brittle fracture. Furthermore, at cryogenic temperatures the presence of sealants in interthread spaces of the flanges may only adds to the complexity of the situation.
One known method of sealing used for repairing containers for fluids consists of applying a sealing gasket or patch on the surface to be repaired. The patch is comprised of alternating layers of a reinforcing material impregnated with a polymerizable binder and a foamed plastic, the pores of which are filled with a curing agent. After the curing agent is displaced from the pores onto the binder, the patch is pressed against the surface to be repaired (see RU Patent Application 95116511, C09K 3/10, C09J 5/00, F16L 55/175, published Sep. 27, 1997).
It should be noted that in addition to such drawbacks as complexity, unreliability, inapplicability for sealing flange joints of intricate geometrical configurations, the patch-application method is absolutely unsuitable for use in pipelines that transport cryogenic fluids. RU Patent 2229417, B63B 43/16 published May 27, 2004 discloses a method of sealing punctured containers such as punctured oil tanks and pipelines that are maintained under live pressure. The method is intended for rescue works on river and sea vessels. The method is based on forming an ice sealant between the hull of the damaged vessel over the entire periphery of the hole and a <<patch>> applied onto the hole. The <<patch>> is comprised of a rigid casing with a system of tubular heat exchangers on the periphery. A cryogen (preferably, liquefied nitrogen) that flows through heat-exchanging tubes produces a cooling effect that forms ice in the areas of contact between the hull and the “patch” whereby the <<patch>> is frozen on the surface of the hull around the punctured area.
Although the aforementioned method is attractive due to its safety, it is still complicated and unreliable. Furthermore, the method is applicable only to patching holes on simple flat or curved surfaces but is inapplicable for sealing leakages through gaps and holes in bodies of complex geometrical configuration, such as, e.g., flanged joints of cryogenic pipelines widely used in aerospace industry.
It is understood that continued leakage of the cryogenic fluid may cause explosion or fire.